The present invention relates to a vapor recovery system for recovering gas that evolves from a source of liquefied gas.
When liquefied gas such as liquefied natural gas is stored in bulk, it is generally stored in very large, highly insulated tanks at very low temperatures and at slightly higher than atmospheric pressure. Heat transfer through the insulation, and heat generated from various processes, causes the liquefied gas to boil or vaporize raising the pressure within the storage tank. The gas is generally removed and recovered by a small boil-off compressor to maintain a relatively constant pressure in the storage tank.
When a large vessel such as a ship unloads liquefied gas into a storage tank, the vaporization rate of the liquefied gas is much greater than under normal storage conditions. Large vapor recovery apparatuses in such systems have also been employed. However, economics often do not justify the cost of increasing the storage tank pressure or installing a large vapor recovery system to handle the high vaporization rates involved with large transfer processes such as a ship unloading process. Such a system would be expensive to install and would be used only during an unloading procedure. Depending upon the frequency of liquefied gas transfer procedures and the value of the vapor recovered, a need has arisen for a system for vapor recovery that has both low investment cost and low operating cost.
It is a feature of the present invention to provide a vapor recovery system for recovering gas evolved from a liquefied gas source, which preferably has low investment cost and low operating cost.
The present invention relates to a method for recovering gas that evolves from a source of liquefied gas. The method includes the steps of providing a source of liquefied gas from which gas evolves, and compressing the evolved gas with a compressor powered by a turboexpander. The compressor forms a source of compressed gas. The compressor is preferably a high speed cryogenic centrifugal compressor; however, other compressors could be used as well.
The present invention also provides a system for the recovery of gas. The system includes a container for a source of liquefied gas from which gas evolves, a compressor in communication with an outlet of the container wherein the outlet enables the withdrawal of gas from the container, and a turboexpander drivingly connected to the compressor for powering the compressor. Herein, the phrase xe2x80x9cdrivingly connectedxe2x80x9d refers to a connection between a turboexpander and a compressor whereby mechanical power from the turboexpander is transmitted to the compressor. The compressor is capable of increasing the pressure of withdrawn gas from the container and forming a source of compressed gas. The compressor includes a work shaft and the turboexpander also includes a work shaft drivingly connected to the work shaft of the compressor. In such an arrangement, power is transmitted from the work shaft of the turboexpander to the work shaft of the compressor.
Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention. The objectives and other advantages of the present invention will be realized and obtained by means of the elements and combinations particularly pointed out in the written description and appended claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide a further explanation of the present invention as claimed.